layers: PR1276, Fix Clang compiler warnings

fixes
- error: using namespace directive in global context in header
- error: comparison of integers of different signs: 'uint32_t'
         (aka 'unsigned int') and 'int'

Change-Id: If56d5b9c33f0de813fbb67120650a887eb9368b0
diff --git a/layers/unique_objects.cpp b/layers/unique_objects.cpp
index b8564f3..d4aabdf 100644
--- a/layers/unique_objects.cpp
+++ b/layers/unique_objects.cpp
@@ -260,7 +260,7 @@
                                                    "Google Validation Layer"};
 
 static inline PFN_vkVoidFunction layer_intercept_proc(const char *name) {
-    for (int i = 0; i < sizeof(procmap) / sizeof(procmap[0]); i++) {
+    for (unsigned int i = 0; i < sizeof(procmap) / sizeof(procmap[0]); i++) {
         if (!strcmp(name, procmap[i].name))
             return procmap[i].pFunc;
     }